Estimation of fungal infection of peanut kernels by determination of free glutamic acid content

Abstract

Peanut kernels (Tainan 9, a Spanish cultivar) inoculated with Aspergillus parasiticus, A. flavus, A. niger, or A. ochraceus as well as noninoculated kernels were incubated in a humidified environment (relative humidity, 100%) at 25°C for 7 weeks. Internal fungal populations and changes in moisture and sucrose content and free amino acid composition of the kernels were determined periodically. Fungal populations determined by using A. flavus and A. parasiticus agar and rose bengal chlortetracycline agar as enumerating media were closely correlated. Moisture content in the kernels increased from 5.8 to 20.4% (dry basis), and changes in individual free amino acid contents varied, depending upon the incubation time and type of fungus used as an inoculum. In the early infection period (up to 5 weeks), sucrose contents and logarithms of threonine and tyrosine contents increased while logarithms of free glutamic acid content decreased linearly with incubation time. A negative linear relationship was further obtained between logarithms of fungal populations and the logarithm of free glutamic acid content (R2 > 0.80) of the infected peanut kernels.